Dear Mr. Kosik,

the maximum density of andesite I read about is 2,8 g/cm³. How high is your measured density? What was your procedure to measure the density? What kind of density did you measure (dry density, wet density, density at standard conditions)?

Best Regards

Andesites with it's normal mineralogy generally have densities between 2.6 to 2.8. The higher density measured may imply presence of some accessory mineral having higher specific gravity or it could be due to higher concentration of any specific accessory.  it can be even due to presence of some oxide or sulphide ore in good quantities. Check the mineralogy. the following link gives details about rock densities.

 At least 3.1 g/cm³. I measured it by gas pichnometer. There are no ore minerals, but olivine, cpx, opx, plagioclase and oxides in small amount... 

Dear mr. Kosik,

are you sure that you analysed an andesite and not by mistake something else? According to the mineralogical composition you could also have analysed a gabbro or peridotite. These rocks have densities of around 3,1 g/cm³ or higher.

Best Regards

100% sure that  it is not plutonic. It is vesicular and was formed during near surface explosive activity

Interesting. Some pictures of the sample and of the outcrop? From where does the sample come?

Regards

Try this very useful software, recently published in G3.

Michele

Adding to Christian Gunther's comment, the mineralogy could indicate andesite, but could also suggest basalt or a more basic extrusive rock. Most andesites do not contain olivine. What is the anorthite content of the plagioclase? Does the rock also have a glassy groundmass or is it completely crystalline?

Yes, it was probably a basalt, because the plagiclase crystal have Ca-rich core and Na-rich rim... 

Yes, but it is strange having 58% SiO2. All I can imagine is a basic magma rich in olivine, pyroxene's and calcic plagioclase mixing with a silicic magma. The high density is still unexpected.

Dear Mr Kosik

SiO2 weight % in andesite is between 52 and 65 you having 58%SiO2 .It is nearly basic magma. It can have more felsic mineral such as olivine and pyroxine group. So it density is a more than normal density.

Dear Szabolcs: You don't specify which is this "extreme high density" you measured in this andesite. Soe Min says that a SiO2 between 52-65 is the normal range for andesites, you say your sample has 58%, but I don't think it is a basic magma, but an intermediate one, basic magma has SiO2 between 45-52%, like basalt. A rock like the one you mention cannot have a very high density, but an intermediate density, around 2,85 g/cm3, depending in its H2O percentage, of course. Which mafic minerals are present in the andesite, augite, hypersthene, hornblende, biotite, or a combination of them? How altered is the plagioclase, is it stongly saussuritized or sericitized? Does it have a glassy groundmass, is it devitrified? With 58% silica it could be an alkaline lava, a hawaiite or mugearite, perhaps, which can easily confused with a true orogenic andesite. If you don't specifify all these things, it is impossible to give you any advise! Truly, Sebastian.

The mafic minerals are olivine, augite and hyperstene. Plagioclase crystals are fresh. The rock supposed to be calc-alkaline. Groundmass is quite microlithic. Density is around 3.1 g/cm3.

Olivine is quite rare in andesite (I would say that It is extremely rare or never found). Only boninites (a sort of MgO-rich andesites) can have high amounts of olivine. More than the SiO2 content you should consider also the sum of alkalis to classify this rock as andesite. In case it falls in the alkaline field you should also consider the (Na2O-2)/K2O ratio to classify it as as a latite or benmoreite (but not an hawaiite).

A last comment is on your phrase "the rock supposed to be calc-alkaline". Despite it is widely used, this adjective in very often misused. How did you classify this rock as calcalkaline?

The question is a non-starter, I am sorry to say. The density in a rock is determined by its modal mineralogy. In the case of a fresh andesite with 58% SiO2, the proportions of plagioclase (and its An content), Quartz (if present), pyroxene,  amphibole, biotite and oxide phase would have to be taken into consideration. 

Dear Szabolcs: now you're giving more information about this strange rock you are studying. Certainly if it contains modal hypersthene cannot be an alkaline rock, augite is quite common in calc-alkali rocks, and the presence of olivine is not impossible in boninites and adakites too. Still you don't specifiy the %An in the plagioclase, you say it is fresh, and also don't mention any glasy groundmass. A density of 3.1 is indeed a bit high for an andesite, is more typical of basalts, picrites, and olivine rich-rocks. How much olivine does this "andesite" contains? Also, in which tectonic setting is it found, which is its age, location, which other volcanic rocks are associated to it, and so on?

Truly, Sebastian.

I had chance only for SEM examination yet, so I do not know all the geochemical properties... 

Assuming that the rock is holocrystalline, an andesite should contain less than 40 modal % of mafic minerals. Plagioclase of andesine range is expected to be the principal component, and considering the SiO2 content, I would not rule out the presence of a silica mineral. With these considerations in mind, a density of 3.1 looks rather high unless there is a lot of opaque oxide and pyroxene.

I once again suggest a careful modal analysis.

Dear Szabolsc: I don't agree in part with M. Qasim Jan answer. If the rock contains olivine it cannot have any silica minerals, except if the silica is present in an non-equilbrated glassy groudmass. If the olivine is cognate to the lava, and it is not xenocrystic, then crystals of trimidite or beta-quartz pseudomorphs cannot form in the same magma, being silica undersaturated. An excess silica could be present under non equilibrium conditions, but only in the groudmass. But this lack of equilibrium would cause alteration of the former olivine, usually as rims or coronas of orthopyroxene..., un less the rock is very young. If the olivine is fresh, then the original magma should have been silica understaturated, and the rock is not an andesite, being alkaline, adakitic or boninitic. You mention there's hypersthene in the rock, but don't specify if this mineral forms individual primary crystals or if it is an alteration of olivine. This would really make a difference! Definitely, detailed thin section petrographic work must be done in this lava, SEM is not enough. We are all eager to see some photomicrographs of it, without and with crossed polars, and with an appropiate graphic scale! Truly, Sebastian. 

I agree, Mg olivine does not coexist with silica minerals. But considering the rather  high SiO2 content of the rock, I am not sure if it really contains olivine. If we can have an idea of the modal composition of the rock, it would help. 

No more comments until some thin section photomicrographs are included. This "olivine" should be evident if it is fresh and unaltered. Everything else is a waste of time. Truly, Sebastian.